The cardiovascular system transports nutrients, hormones, and respiratory gases through circulating blood. The heart generates pressure to move blood through vessels, while capillaries allow substances to exchange between the blood and body tissues.
Blood consists of plasma and formed elements, including red blood cells. Plasma is mainly water, making it an effective transport medium for dissolved substances.
| Substance | Main transport mechanism | Exchange at tissues |
|---|---|---|
| Nutrients | Water-soluble nutrients, such as glucose and amino acids, dissolve in plasma. Most absorbed lipids enter the blood as lipoprotein particles after first passing through the lymphatic system. | Nutrients move from capillary blood into tissue fluid and then enter cells, mainly by diffusion or facilitated diffusion. |
| Hormones | Endocrine glands release hormones into the blood. Water-soluble hormones travel dissolved in plasma, while many lipid-soluble hormones bind to carrier proteins. | Hormones leave capillaries and bind to specific receptors on or inside target cells. Only cells with the appropriate receptor respond. |
| Oxygen | Most oxygen binds reversibly to haemoglobin inside red blood cells, forming oxyhaemoglobin. A small amount dissolves in plasma. | Oxygen dissociates from haemoglobin and diffuses from the blood into tissues down its concentration gradient. |
| Carbon dioxide | Carbon dioxide is transported mainly as bicarbonate ions in plasma. Smaller amounts bind to haemoglobin or dissolve directly in plasma. | Carbon dioxide produced by respiration diffuses from cells into the blood and is transported to the lungs for exhalation. |
Exchange is efficient because capillaries have walls only one cell thick, a large total surface area, and slow blood flow. These features create a short diffusion distance and provide time for exchange. Continuous circulation also maintains concentration gradients between blood and tissues.
A common misconception is that plasma transports all oxygen. In fact, most oxygen is transported by haemoglobin in red blood cells.
In an IB SEHS A.1.3 Transport response, distinguish between the transport medium and the exchange process. Examiners expect named blood components, the form in which each substance is carried, and an explanation of diffusion across capillary walls.
